Centralizer

ABSTRACT

A centralizer ( 101 ) comprises two annular bands ( 102, 103 ) which are spaced apart by a plurality of hollow members ( 104, 105, 106 ) extending therebetween. The hollow members ( 104, 105, 106 ) overlie openings ( 110-115; 116-121 ) in the annular bands ( 102, 103 ) respectively. In use, fluid can enter and leave the hollow member via the openings. This flow helps maintain a film of fluid between the annular bands ( 102, 103 ) and the tubular on which the centralizer is mounted and thus facilitates rotation of the tubular with respect to the centralizer.

This invention relates to centralizers for use in the construction ofoil and gas wells.

During the construction of oil and gas wells a borehole is drilled inthe ground. A string of tubulars is then lowered down the borehole andthe annular space between the tubulars and the borehole filled withcement.

It is important to ensure that the tubulars are held centrally in theborehole during cementation and it is usual to provide the tubulars witha plurality of centralizers which act between the tubulars and theborehole.

One type of centralizer which is commonly used comprises a pair ofannular bands which are spaced apart by a plurality of hollow memberswhich extend therebetween. Such hollow members may extend parallel tothe longitudinal axis of the centralizer or may, as disclosed in ourco-pending application 96 17789.4, be inclined thereto. The hollowmember may be designed to withstand great radial load or to collapseirreversibly when the radial load exceeds a predetermined value. Thecentralizer may, in use, be secured fast on a tubular, or may berotatably mounted thereon and retained against significant axialmovement by stop collars disposed on the tubular to either side of thecentralizer.

This later arrangement is generally used when it is desired to rotatethe tubulars during cementation. Rotation is intended to improve thedistribution of the cement in the annular space between the tubulars andthe borehole and reduce the occurrence of voids when the cement sets. Itis desirable that the tubulars should be free to rotate relative to thecentralizers and conventionally this has been achieved by the simpleexpedient of making the inner diameter of the centralizers slightlylarger than the outer diameter of the tubular. Although this works tosome extent there is still significant friction between the centralizersand the tubulars. If the string of tubulars is sufficiently long thetorque which must be applied to the uppermost tubular to ensure rotationof the entire string of tubulars can exceed the maximum torque permittedto be applied to the threaded joints between the tubular which candamage the joints. This is undesirable.

In order to help reduce this problem the present invention provides acentralizer which comprises a pair of annular bands which are spacedapart by a plurality of hollow members, characterised in that eachannular band is provided with an opening which underlies a hollow memberso that, in use, liquid can flow through the said opening in one of saidannular bands, along said hollow member, and out of the opening in theother of said annular bands.

The hollow member may extend substantially parallel to the longitudinalaxis of the centralizer or may be inclined thereto, for example at anangle of from 30° to 60°, preferably from 30° to 45° to the longitudinalaxis of the centraliser.

The hollow member is preferably of rounded cross-section, although itcould be of any convenient shape, for example rectangular or square.

Preferably, the hollow member has a radial inner surface which issubstantially flush with the radial inner surface of the annular bands.

Advantageously, the hollow member is tapered towards each end.

Preferably, the hollow member will, in use, substantially permanentlycollapse against casing when subjected to a lateral load of from 5 to 15tonnes.

Advantageously, the annular bands are formed in one piece although theycould also be made in two separate pieces which can be mountedcircumjacent casing.

Preferably, each annular band is provided with a plurality of holes eachof which underlies a respective hollow member.

For a better understanding of the present invention reference will nowbe made, by way of example, to the accompanying drawings in which:

FIG. 1 is a side elevation, with parts broken away, of one embodiment ofa centralizer in accordance with the present invention mounted on atubular;

FIG. 2 is a section taken on line II—II of FIG. 1; and

FIG. 3 is a section taken on line III—III of FIG. 1.

Referring to the drawings there is shown a centralizer which isgenerally identified by the reference numeral 101. The centralizer 101comprises a pair of annular bands 102, 103 which are spaced apart by sixhollow members, three of which 104, 105, 106 are visible in FIG. 1.

Each hollow member 104, 105, 106 is inclined at an angle a of frombetween 30° to 60° and preferably from about 30° to 45° to thelongitudinal axis of the centralizer 101.

Each hollow member 104, 105, 106 has skirt portions 107, 108 whichextends radially inwardly and finishes flush with the radially innersurface of the two annular bands 102 and 103.

In use, the centralizer 101 is slid over a tubular 109. The centralizer101 is secured in position by stop collars (not shown) placed above andbelow the centralizer 101.

Each hollow member 104, 105, 106 comprises a thin sheet of steel whichis about 3 mm in thickness and is shaped into a curve. Each hollowmember 104, 105, 106 is also tapered both radially and circumferentiallytowards each end to facilitate movement of the centralizer 101 in theborehole.

As can be seen in the Figures each annular band 102, 103 is providedwith six openings. Thus annular band 102 is provided with openings 110,111, 112, 113, 114 and 115 whilst annular band 103 is provided withopenings 116, 117, 118, 119, 120 and 121. It will be noted that eachopening underlies a respective member. Thus opening 111 underlies theupper end of member 105 whilst opening 117 underlies the lower end ofthe same member 105.

In use, a plurality of centralizers similar to centralizer 101 arerotably mounted between stop collars on a string of casing which is thenlowered down a borehole. When the casing is in place circulating fluidis pumped down the annular space between the casing 122 and the tubular109 and then travels to the surface via the inside of the tubular 109.

As the circulating fluid passes each centralizer 101 the majority passesbetween adjacent hollow members 104, 105, 106. As the circulating fluidpasses downwardly over the annular band 102 between the hollow member104, 105, 106 it is diverted sideways. As it passes over the upper edgeof the annular band 103 a turbulent rolling action is initiated which isamplified as the circulating fluid passes over the annular band 103. Thecombined swirling and rolling action provides an extremely effectiveclearing and scouring action which is highly desirable. However, part ofthe circulating fluid passes through the clearance 123 between theannular band 102 and the tubular 109. Thus as can be seen from thearrows 124 in FIG. 2, part of the flow enters the inside of the member105 via the opening 111 whilst another part enters the member 105 afteremerging from the clearance 123. The flow passes along the inside ofmember 105 before passing through opening 117 into clearance 125.

If the string is rotated the annular bands 102, 103 ride on a film ofcirculating fluid which helps avoid metal to metal contact between theannular bands 102, 103 and the casing. This in turn reduced the frictiontherebeween.

After the annular space between the casing 109 and the borehole has beenprepared cement is pumped down the casing and up into the annular spacewhile the casing is rotated. The cement is then allowed to set in theconventional manner.

Occasionally, part of the borehole will collapse when running casing.The usual procedure when this occurs is to withdrawn the casing, makegood the problem as necessary and reinsert the casing. Although thecasing can usually be withdrawn without too much difficulty the forcesimposed on traditional centralizers often result in their disintegrationwith the result that broken parts of centralizers remain in theborehole. This is most undesirable. Applicants PCT Publication No. WO96/09459 addresses this problem by providing a centralizer with memberswhich have sufficient strength to centralize the casing but which willcollapse if withdrawn through a relatively rigid constriction. Theunderlying principle is that it is better to replace a permanentlydeformed centralizer at the surface rather than to leave parts of adisintegrated centralizer in the borehole. Typically, the members shouldsubstantially irreversibly collapse when subjected to a lateral load offrom 5 to 15 tonnes with 11 tonnes being currently used for designpurposes for most occasions.

What is claimed is:
 1. A centralizer (101) comprising a pair of annularbands (102, 103) which are spaced apart by at least one hollow member(104, 105, 106), wherein each of the annular bands (102, 103) isprovided with an opening (110-112; 116-121) which underlies a hollowmember (104, 105, 106) that is inclined to a longitudinal axis of thecentralizer, so that fluid can flow through the opening in one of theannular bands (102), within the hollow member, and out the opening inthe other of the annular bands.
 2. A centralizer for a wellbore tubularcomprising: a pair of annular bands which are spaced apart by at leastone hollow member inclined to a longitudinal axis of the centralizer,each of the annular bands provided with an opening which underlies ahollow member so that in use, a fluid can flow through the opening inone of said annular bands within said hollow member, and out of theopening in the other of said annular bands, the centralizer rotationallyindependent of the wellbore tubular.
 3. The centralizer of claim 2,wherein the inclination of the hollow member to the centralizer is from30 to 60 degrees.
 4. The centralizer of claim 2, wherein the inclinationof the hollow member to the centralizer is from 30 to 45 degrees.